1. Field of the Invention
The present invention relates to the cambering and tempering of glass sheets. More specifically, the invention relates to a shaping roller having an adjustable radius of curvature and the use of said shaping roller in a cambering-tempering machine in which the shaping bed is constituted by rotary elements arranged along a path whose profile is curved in the glass sheet travel direction. The invention also relates to an operating procedure making it possible to obtain glass plates with a complex shape by using such a cambering machine. The invention more particularly applies to the production of glass sheets for cars, which are cambered and tempered.
2. Description of the Related Art
French patent FR-B-2442219 discloses the cambering of glass sheets which have previously been heated to the softening point by making them travel on a shaping bed which has a terminal tempering zone and is constituted by rotary elements arranged along a curved profile path, usually a circular profile with a preferably upwardly turned concavity. The radius of the path of the rotary element corresponds to the radius of curvature given to the glass sheet in its direction parallel to the direction of travel. An important improvement in this process, known from European patent EP-B-133114, consists of providing a discharge conveyor whose first member is constituted by the final member of the shaping bed and which has a different slope from that of the bed, a tilting member also being associated with said first member of the discharge conveyor. Installations of this type make it possible to produce car glass plates at exceptionally high speeds because the glass sheets are able to follow one another very closely and with a high optical quality due to the quasiuniform support of the entire surface of the glass sheet throughout the shaping operation.
However, the hitherto known industrial constructions are usually limited to the production of cylindrical glass plates, i.e., those obtained by rotary members constituted by unbent or undeflected, straight rods. However, in addition to the demand for cylindrical glass plates, e.g., those to be used for side windows equipped with a window winder, there is an ever growing demand for glass plates having a more complex shape including a second curvature in a direction perpendicular to that of the main curvature.
French patent FR-B-2442219 discloses using, for the production of such glass plates with two directions of curvature, a cambering process on a shaping bed having a curved profile and constituted by bent rotary members, the glass sheets then travelling between the two rotary members having complementary shapes. The literature essentially discloses three types of bent rotary members, namely double cone-like rollers associated with spindle-shaped rollers, guide-type rollers and cambered rollers.
By definition, the diameter of a double-cone or spindle-type roller varies constantly over the entire roller length. Therefore the peripheral speed of the roller is variable over the entire roller length and the drive speeds of the lateral parts of the glass sheets and the central parts are different. These speed differences make it impossible to treat glass plates whose leading edge is not parallel to the axis of the roller and the sides perpendicular to said axis. In addition, to the central bulging part of the spindle corresponds the central thin part of the complementary double-cone structure. In other words, the drive speed of the glass due to the lower roller and the drive speed due to the corresponding upper roller then differ. In the direction of the axis line, the driving due to the spindle can be twice as fast as the driving due to the double-cone. This is not favorable to the optical quality and also requires delicate settings for each new installation. In addition, an invariable transverse radius of curvature is imposed by the shape of the double-cone or the spindle, so that it is only possible to produce a wide range of glass plates by accepting prohibitive costs.
The second type of rotary member is constituted by guide-type rollers which are formed by bent rods coated with axially deformable, rotary tubular envelopes, but which are rigid in rotation. The envelopes rest on the bent rods via slip rings and are covered by a protection, e.g., by winding silica threads or by a sheathing with a knitted or woven structure of silica or other refractory fibers. By varying the inclination of these guide rollers, in the manner known, e.g., from French patents and certificates of addition 1476785, 92064, 2129919, 2144523, 2189330, it is possible to modify the radius of curvature of the bed defined by them. The guide rollers have a constant diameter and consequently their peripheral speed is constant over their entire length, which represents a considerable advance compared with double-cone or spindle-type rollers. However, when applied to a shaping bed with a curved path, said guide rollers impose a minimum transverse curvature, because they do not permit the creation of a purely cylindrical shaping bed, which is obviously due purely to geometrical considerations. In addition, these guide rollers must be spaced by at least 50 mm due to the overall dimensions of the envelopes and the slip rings, and also due to the space necessary for inclining said guide rollers, which leads to a possibility of the glass sheets sagging between them, which is prejudicial to the optical quality.
The third and final type of rotary member, namely the cambered roller, does not suffer from any of these major defects, at least in theory. As it has a constant diameter, or one which can at least be considered as such, a cambered roller has a peripheral speed equal at all points of its surface. In addition, the deflection given to it can obviously be zero, which makes it possible to form a cylindrical shaping bed.
However, by their very design, the known cambered rollers are not suitable for producing a shaping bed along a curved profile path. German patent application DE-A-3310357, e.g., discloses a roller constituted by a cambered, fixed, central bar surrounded by an envelope, which is rotated.
French patent application FR-A-2604700 also discloses a conveyor roller constituted by a fixed, central bar cambered by means of a holding device, which vertically displaces the center of the bar. In addition, the bar is surrounded by sleeves about which are fitted freely rotating glass holders. Thus, the principle is the same as that described in connection with the guide rollers with a dissociation between the moving parts driving the glass sheet and the fixed parts defining the shape of the roller and therefore that given to the glass sheet. This principle is again described in French patent application FR-A-2572387.
Thus, the prior art means are not true cambered rollers, but are systematically constituted by a stationary, cambered, central bar and members rotating about said bar. Thus, the relatively large minimum spacing between the rollers required for guide rollers is also necessary here.
Moreover, the means used for the bending of the central bar are not free from disadvantages. Thus, according to FR-A-2572387 the bar is wedged or jammed between bearings by adjusting its position by means of a series of threaded bushes, which involves an extremely long setting or adjustment time for each production change in order to obtain precisely the same radius of curvature for all the rollers, no matter whether they are located above or below the glass sheet and with degrees of wear to the parts which can obviously differ from one roller to the next. The apparatus proposed by FR-A-2604700 permits a rapid adjustment of the radius of curvature but, as stated hereinbefore, uses for this purpose a holding device acting on the center of the bar or at least on a part of the roller above which travel the glass sheets. Thus, such a device cannot be used with a bar combining the functions of defining the shape and deriving the glass sheet.
Finally, according to DE-A-3310357, the end of the bar not equipped with the rotating device for the envelope is guided by means of two rolls between which is gripped the bar and which are mounted at the apices of a triangular support plate, in which the third apex is articulated about an axis perpendicular to the plane containing the center of the roller. The plate and the rolls occupy a large volume above and below the roller because the rolls must be dimensioned so as to withstand very high forces, so that it is not possible to superimpose two cambered rollers unless they are spaced by the thickness of a glass sheet, which is typically between 2 and 6 mm.